JPS589786B2 - Manufacturing method of silicon carbide ceramic - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention is based on thermal open molding (by hot pressing) in the atmosphere.
The present invention relates to a method for manufacturing uniformly densified silicon carbide ceramic.

Silicon carbide has excellent oxidation resistance, corrosion resistance, and
Due to its excellent thermal conductivity, low coefficient of expansion, excellent thermal shock resistance, and excellent wear resistance, it is used as components for gas turbines, check valves and sealing materials for corrosive liquids, heat exchangers for high-temperature furnaces, Furthermore, it is seen as promising for a wide range of applications such as wear-resistant parts.

Conventionally, dense silicon carbide ceramics have been manufactured by reaction sintering, chemical vapor deposition, hot pressing, etc. However, it is difficult to obtain a density close to the theoretical density of silicon carbide, 3.21 g/cm3, and hot pressing is used to produce the theoretical density. 98% (3.
15 g/cm3) or more can be experimentally obtained.

Hot pressing of silicon carbide has been studied for a long time, and A
lliegro (J. Am, Ceram. Soc., 3
9 [11] 1956) investigated various sintering aids and found that aluminum (Al), boron (B), and iron (Fe)
It has been shown that a sintered body having a theoretical density of 98% can be obtained by adding the following.

Recently, products using Al or Al compounds such as Al2O3 as additives (JP-A-49-7311), B-B4C
(Japanese Patent Application Laid-Open No. 49-99308)
JP-A-50-34608) was published, and a high-density body of more than 98% was obtained, but both of the above-mentioned Alliegro
The use of finely pulverized submicron silicon carbide powder and the high pressure ratio of the hot press made densification easier.

However, in reality, it is not easy to obtain submicron silicon carbide powder; it requires mechanical crushing and classification using a ball mill, etc., and further processing such as removing impurities mixed in during crushing, and the high pressure of a hot press. However, there are also limitations on the strength of the graphite mold used as the molding material, making it impractical.

By the way, in the hot press of silicon carbide, Al and Al
When an Al compound such as 2O3 is used as an additive, the sintering mechanism is due to the solid solution of Al in the silicon carbide crystal lattice, which makes it difficult for particle growth to occur during densification, resulting in a high-strength compact. However, only in the case of highly purified β-SiC powder as raw material silicon carbide powder or 6H type α-SiC powder with a very small amount of Al solid solution, the densification effect is promoted in hot pressing. Although,
Hot pressing of 4H type or 15R type α-SiC powder, in which a considerable amount of Al is already dissolved as an impurity in the silicon carbide crystal lattice, no longer shows a densification effect.

Therefore, the raw materials for hot pressing silicon carbide using Al-based additives are limited to highly refined β-SiC powder and 6H-type α-SiC powder that have almost no Al dissolved in the crystal lattice. It has its drawbacks.

Furthermore, in hot pressing using B or a B compound such as B4C as an additive, the sintering mechanism is a solid solution of B in the silicon carbide crystal lattice.

Unlike the case of Al-based additives, it exhibits a densifying effect regardless of the type of SiC, making it an excellent sintering aid for SiC.

However, contrary to the case of Al-based additives, it has the disadvantage that it promotes grain growth between SiC particles during densification, resulting in a decrease in the strength of the SiC sintered body.

Therefore, in order to suppress this particle growth, we used submicron β-SiC powder and added a small amount of carbon to produce 700 kg of powder at a temperature below 2000°C in an inert atmosphere.
Hot pressing at a high pressure of /cm3 has been proposed, but it is difficult to obtain submicron β-SiC powder, and due to the strength of graphite molds for hot pressing as silicon carbide ceramics become larger. High pressure molding becomes difficult,
Furthermore, it requires complicated and expensive equipment to maintain an inert atmosphere, making it difficult to put it into practical use.

The present invention has been made in view of the above-mentioned circumstances, and it is an object of the present invention to provide a method for manufacturing a uniformly dense silicon carbide ceramic, which solves all the conventional drawbacks at once.

The present invention will be explained in detail below.

According to the present invention, in the hot pressing of silicon carbide, commercially available α-SiC polishing powder is used, the excellent densification effect of the B compound additive is utilized, and
temperature of ~2200°C and at least 100 kg/c
m3 low pressure (preferably 100-300 kg/cm3
Provided is a method for producing high-density silicon carbide in which SiC particle growth is extremely suppressed under pressure of

That is, an appropriate amount of B2O3 or H3BO3 and graphite powder or carbon powder is added to commercially available abrasive powder α-SiC (of course β-SiC powder can also be used) with an average particle size of about 1 to 5 μm as SiC powder, and the mixture is dried for about 1 hour using a ball mill. By mixing, raw material powder for hot pressing is prepared.

Here, each addition amount is 10 to 20 W% of B2O3 (H3B
In the case of O3, the carbon content is 6 to 12 W% (in the case of carbon compounds such as tar pitch resin, the carbon content after carbonization is 6 to 12 W%).
), preferably carbon vs. B2O3
The molar ratio of 3 to 3.5:1 (0.5 to 0.0 by weight)
6:l).

This is B2O3 according to the order (■→■→■) shown in the following formula.
This is because B and C react most efficiently to produce B or B4C, which acts as a sintering aid.

The reaction formulas from ■ to ■ indicate that B203 (or H3BO3) is generated during the heating process during hot press molding.
liquefies in the temperature range of 460 to 1860℃, and liquefies in the temperature range of 1800 to
From a temperature of 1850°C, CO gas generation becomes remarkable, and 1
This reaction formula was estimated and derived from the phenomenon that almost completes at around 900°C, and was confirmed.

Incidentally, in hot pressing of silicon carbide to which powders of B and B4C are added, sintering starts at 1850 to 1900°C.

Therefore, in the present invention, B and B4C as sintering aids are generated just before the start of sintering, and moreover, because they are generated from liquid B203,
Since B and B4C are uniformly diffused within the compact and are much more reactive than powdered B and B4C, their effectiveness as sintering aids is further promoted.

Furthermore, an important feature of the present invention is that the added B2O3 is
This is a phenomenon that erodes SiC at temperatures above 00°C.

This erosion phenomenon is recognized by the fact that the weight loss after hot pressing is greater than the added amount, as shown in FIG. 1, and by the fact that the particles have become extremely fine from the etching photograph of the hot press molded product.

Furthermore, in order to clarify this erosion reaction, we conducted X-ray diffraction analysis of the molded body, but no substances other than SiC were detected, and no HF was detected.
Even when immersed in an aqueous solution, no loss in dissolution of glassy materials such as SiO2 or borosilicate was observed.

Therefore, the molded body was found to be a high-purity SiC body,
This erosion is presumed to be due to the following reaction occurring on the surface of the SiC particles, and it was found that the products were dissipated in the form of gas.

As a result, the SiC particles become finer due to the erosion action of B2O3, and their active surfaces are exposed, so that the sintering action is further promoted.

However, the key factor for making this feature effective is the heating rate, and the sintering start temperature (1800-1900
It is desirable to raise the temperature as quickly as possible to (1°C).
100°C/min from 000°C to 1800°C).

This is because the erosive action of B2O3 is suppressed only on the SiC particle surface, and the B2O2 gas reacts with the added carbon without escaping before the start of sintering, and the B2O2 gas of the sintering aid
This is to make it act effectively as a source.

A further important feature of the present invention is that SiC of liquid B2O3
This is a lubricating effect between particles.

Normally, hot press equipment applies pressure by one or both sides, and in the case of powder samples, there is a phenomenon of pressure disproportionation in the sample, and in particular, due to the influence of side friction with the mold material, the hot press molded product Due to insufficient pressure, areas with a low degree of densification occur.

According to the present invention, the additive B2O3 is liquefied at low temperature (m・p
During this time, the SiC particles are completely covered and lubricated to maintain the liquid state up to the sintering starting temperature (boiling point of B2O2 is 1860°C).

Therefore, this oil sliding action has a major feature in that the phenomenon of pressure disproportionation is eliminated.

However, to make the present invention even more effective, mold material (graphite)
It is preferable to coat the mold release material with a mold release material to prevent elution of liquid B2O3 and reaction with the graphite mold material.

As described above, according to the present invention, it is possible to adjust and highly purify the submicron particles of raw material SiC, which was a problem in conventional hot pressing of silicon carbide, and to uniformly mix additives as sintering aids during sintering. The prevention of particle growth, etc. has been resolved, and it can be obtained at a cost less than 1/10 of submicron-grade SiC powder, instead of using the impractical method of high-pressure hot pressing at 2000°C or less and 700 kg/cm2 in an inert atmosphere. Using commercially available polishing powder, and in an atmospheric atmosphere,
A possible and practical manufacturing method was obtained that allows obtaining a high-density SiC molded body at ~2200° C. and a low pressure of at least 100 kg/cm 2 .

In addition, the density and B2O of the molded body (SiC) made by the present invention
Fig. 2 shows the results of a research experiment showing the relationship with the amount of 3 etc. added.

As can be seen from FIG. 2, according to the present invention, a molded body (SiC) with very high density can be obtained.

In other words, it is confirmed from the experimental results shown in Figure 2 that when the molar ratio of carbon to B2O3 is 3.5:1, the density of the silicon carbide compact is increased the most compared to other molar ratios. and the amount of B2O3 added is 1
A range of 10 to 20 W% with a peak of 5 W% is the point at which the density of the silicon carbide molded body can be increased.

From this, the amount of carbon to be added is determined by the molar ratio (3.5 to 1
) and atomic weight (carbon≒12.0, B2O3≒69.6)
It was found that the content was 6 to 12% by weight, and it was confirmed that the density of the silicon carbide molded body could be increased most with such a blend.

Furthermore, the present invention completely eliminates the need for a non-oxidizing atmosphere that requires complicated operations and expensive equipment, making it possible to carry out thermal open molding in an atmospheric atmosphere.

However, in the present invention, as mentioned above, B is used as a sintering aid.
The 4C production reaction proceeds regardless of the presence or absence of a non-oxidizing atmosphere, and is possible even in a non-oxidizing atmosphere.

As described in detail above, the present invention uses inexpensive commercially available silicon carbide powder having an average particle size of several microns.
% by weight of boric anhydride (B2O3) or 15-30% by weight of boric acid (H3BO3) which becomes boric anhydride by heating and dehydration.
) and carbonaceous powder adjusted to have a carbon content of 6 to 12% by weight, and then
We have obtained a method for manufacturing high-density silicon carbide ceramic by hot pressing at a temperature of 2200°C and a low pressure of 100 to 300 kg/cm2, and furthermore, in this hot open molding process, In particular, when the temperature is raised, boric anhydride (or boric acid) liquefies at a relatively low temperature and is uniformly dispersed and infiltrated into silicon carbide powder and carbonaceous powder, and reacts with the added carbon source to uniformly carbonize as a boron source. It is also a manufacturing method that includes a step of diffusing into silicon powder as a solid solution, and a step of eroding the surface of silicon carbide powder to make the silicon carbide powder finer and completely suppressing the particle growth of silicon carbide. obtained a method for producing a substantially uniform and highly dense silicon carbide ceramic by increasing the rate of temperature rise up to a predetermined temperature, that is, the sintering start temperature.

Since these manufacturing methods according to the present invention are practical and can be industrially produced at low cost, it is believed that the silicon carbide ceramics obtained will open the way to a wide range of industrial applications.

[Brief explanation of drawings]

Figures 1 and 2 are charts showing an example of the results of research experiments related to the present invention. The figure shows the relationship between the density of a silicon carbide molded body and the amount of B2O3 added.

Claims (1)

[Claims] 1 Silicon carbide powder having an average particle diameter of 1 μm to 5 μm is mixed with 10 to 20% by weight of boric anhydride (B2O3) or 15 to 35% by weight of boric acid (H3BO3) and adjusted to have a carbon content of 6 to 12% by weight. A method for producing a high-density silicon carbide ceramic by adding and mixing a carbonaceous powder obtained by adding the carbonaceous powder obtained above, and thermally opening molding the mixture in an air atmosphere at a temperature of 2000 to 2200°C and a low pressure of 100 to 300 kg/cm3.